1. Field of the Invention
The present invention relates to an electrode material for a lithium secondary battery, an electrode structure using the electrode material, and a lithium secondary battery using the electrode structure. Incidentally, the designation of groups of elements in the present claims and specification is in conformity with the designation adopted in the IUPAC Periodic Table.
2. Related Background Art
In these years, the amount of CO2 gas contained in the atmosphere has been increasing, and there is an apprehension that the green house effect resulting therefrom leads to the global warming; thus, the countermeasure for reducing the exhaust amount of CO2 gas is being investigated on a global scale. For example, a thermal power plant where a thermal energy obtained by burning fossil fuel is converted to an electric energy discharges a large amount of CO2 gas, and accordingly, it has been becoming hardly possible to newly construct a thermal power plant. Under such circumstances, in order to meet the growing demand for electric power, the so-called load leveling has been proposed in which, as a measure for effective use of electric power, the load is leveled in such a way that the nighttime power, namely, the surplus power is stored in the batteries installed at homes and is used in the daytime where the power demand is large. On the other hand, automobiles that run using fossil fuel emit, in addition to CO2 gas, Nox, SOx, hydrocarbons and the like, and hence are considered to be problematic as another source of release of air pollutants. From the viewpoint of reducing the air pollutant release sources, electric automobiles that run by use of a motor driven with an electric power stored in a secondary battery do not emit air pollutants, thereby attract attention, and have been intensively investigated and developed for early practical application. Secondary batteries for use in the load leveling and in the electric automobiles are required to be high in energy density, long in operation lifetime and low in cost.
Furthermore, as for secondary batteries to be used in portable equipments such as notebook personal computers, word processors, video cameras, mobile phones and the like, early provision of those secondary batteries which are compact, lightweight and of high-performance is eagerly demanded.
Various types of high-performance secondary batteries corresponding to the above-described requirements have been proposed, some of which have come into practical use. A representative configuration of such practically applied lithium secondary batteries is such that carbon is used for the negative electrode material (negative electrode active material) constituting the negative electrode, a lithium cobalt composite oxide (LiCoO2) is used for the positive electrode material (positive electrode active material) constituting the positive electrode, and an ethylene-carbonate-based electrolyte is used for the electrolyte. The lithium cobalt composite oxide (LiCoO2) used as the positive electrode material has a drawback that it contains cobalt, which is a nonabundant resource, as its source material and is therefore expensive, but is regarded as having a highest practical value of the existing positive electrode materials (positive electrode active materials). This is mainly because no other alternatives to replace the lithium cobalt composite oxide have been provided yet. More specifically, for example, in contrast to the existing problem such that lithium nickel composite oxide (LiNiO2) which is promising for attaining a high capacity and a high energy density and lithium manganese composite oxide (LiMn2O4) which uses abundant resource as source materials and is therefore inexpensive are difficult to synthesize and cause a lowering in capacity by repeating charging/discharging, the lithium cobalt composite oxide (LiCoO2) is easily synthesized and shows a relatively small lowering in capacity even when repeating charging/discharging. Additionally, lithium nickel cobalt composite oxide (LiNi1-xCoxO2) developed for the purpose of overcoming the above-described drawback of lithium nickel composite oxide (LiNiO2) is still not sufficient in the characteristic of retention of capacity after repeated charging/discharging and is not therefore superior to the lithium cobalt composite oxide (LiCoO2).
Incidentally, for the positive electrode material (positive electrode active material) used for the positive electrode of a lithium secondary battery, there have been proposed a large number of lithium/transition-metal composite oxides each comprising a crystal system composed of a plurality of different species of elements. For example, Japanese Patent Application Laid-Open No. H03-219571 discloses a positive electrode active material in which a part of Mn of LiMn2O4 is replaced (or substituted) with Co, Cr or Fe. Additionally, Japanese Patent Application Laid-Open No. H04-188571 discloses a positive electrode active material in which a part of Co of LiCoO2 is replaced with Ti, V, Cr, Mo, Ni or Fe. Although the techniques involved in these applications are intended to improve the characteristics as the positive electrode active materials by substituting parts of the constituent elements of LiMn2O4 or LiCoO2 with different species of elements, these positive electrode active materials cannot be said to be sufficient in the characteristic involved in the capacity retention after repeated charging/discharging.
Additionally, Japanese Patent Application Laid-Open No. H08-222219 discloses a positive electrode active material obtained by coating the surface of particles of a lithium/transition-metal composite oxide that is a positive electrode active material with TiB2, TiC, TiN, TiSi2, Fe3O4, IrO2 or Au. Additionally, Japanese Patent Application Laid-Open No. H08-250120 discloses a positive electrode active material obtained by coating the surface of particles of a lithium/transition-metal composite oxide that is a positive electrode active material with TiSe2, TiTe2, TiS2 or MoS2. In these materials, by coating with a predetermined material the surface of the particles of the main positive electrode active material, it is intended to improve the characteristics of the surface of the particles of the positive electrode active material. However, even in such positive electrode active materials with the surface-coated structures, the characteristics involved in the capacity retention after repeated charging/discharging cannot be said to be sufficient.
LiMO2 (M represents Co or Ni), which is a lithium/transition-metal composite oxide having the α-NaFeO2 structure, is evaluated to be useful as a positive electrode active material of a lithium secondary battery. However, LiMO2 is liable to cause capacity degradation accompanying charging/discharging and particularly has a serious problem that repeated charging/discharging at a temperature as high as 60° C. leads to a greater capacity degradation, so that LiMO2 is not considered to be worthy of practical use.